Adsorption may be attributed to several factors acting separately or simultaneously. With respect to the adsorbent, one can distinguish between physical and chemical adsorption, and with respect to the adsorbate distinction can be made between polar and non-polar adsorption. The most important force resulting in physical adsorption is called the dispersion force, resulting in attraction between two atoms because of the distortion of the electron density in one atom, which induces a corresponding electrical moment in the atoms of the neighborhood. In chemisorption, a chemical compound is formed between adsorbent and adsorbate, and in contrast to physical adsorption, electron transfer always occurs.
It was found by Lewis et al Ind Eng. Chem. 42, 1319 (1950) that for ideal mixed adsorption, relative adsorbabilities behave like relative volatilities. In cases when relative adsorbabilities approach unity, the relationship between the gas phase and the adsorbed phase concentrations approach linearity, as it is the case for the simultaneous adsorption of ethane and ethylene on activated carbon. The term quasilinear used hereinafter means that the isotherm of one component under no circumstances would cross the isotherm of the other component. Referring here to distillation, a gas mixture which cannot be characterized by linear or quasilinear isotherms would be similar to an azeotrope mixture.
Gas separation is a major problem in the chemical industry. Separation of gases may be accomplished by fractional distillation, solvent extraction, selective adsorption and similar processes. For example, ethylene purities up to 99.9 percent are being produced by low temperature distillation. Usually 50-90 plates and reflux ratios of 4-6 are required depending on the composition of the feed. It was shown by Szirmay, Trans.IchemE., 56, 101(1978), that a similar separation could be achieved in one adsorption column of barely more than one foot in length. Said reference further led to the realization of the possibility of completely separating in one operation a group of the most adsorbing components of a gas mixture, provided this gas mixture is characterized by linear or quasilinear properties. To achieve this effect a component (or components), with the highest adsorbability of said gas mixture, can be used as an agent to cause rectification, such that from said group of the most adsorbing components, the component with the lowest adsorbability can be collected first, in fair purity, next the component with the next higher adsorbability can be collected in fair purity, similarly the remaining components of said group can be collected in fair purities in the order of increasing adsorbabilities, finally the highest adsorbing component (or components) of said group, designated before as rectifying agent, can be collected in high purity. This operation will become more apparent as the specification proceeds.
My prior U.S. Pat. No. 3,727,376 discloses a moving adsorbent bed process.
The present invention provides a new preferential adsorption method and apparatus for separating gaseous mixtures of materials characterized by linear or quasilinear adsorption isotherms.
Another object of the invention is to provide a relatively inexpensive method and apparatus for use in separating specific gases from a feed mixture of gases and separating a specific gas such as ethane from ethylene.
Another object of the invention is to provide rectification effect by using a desired bottom product as rectifying agent.